This invention relates generally to the art of plastic film forming extrusion machines and particularly to a sensing of the nip size as a means of determining the thickness of plastic material being extruded.
Numerous devices have been developed in an attempt to determine nip size and thus allow for adjustments which will assure a constant thickness of material emerging from extruders. Current practice uses a gaging system located downstream of the nip to measure the thickness of the material. This information is fed back to the operator who can then make speed adjustments to compensate for thickness variations detected by the gaging system. Systems are also available to automatically adjust the process on the basis of the thickness measurements.
Other systems have been developed which monitor the pressure on the material being extruded and adjust the nip size in response to changes in pressure.
Examples of such pressure-sensitive nip controllers are disclosed by Smith U.S. Pat. No. 3,135,018 and by Woechener et al. U.S. Pat. No. 4,214,857.
But, no system has been developed which correctly measures the nip size itself so that accurate adjustments can be made.